The D3x is usable until ISO1600, but then drops in IQ dramatically. You can use Software to denoise the ISO3200/6400 Files, but the result is far from good, compared to newer Sensors, like D4 or even D3s. ISO100 is very clean and accurate in color, but outperformed by newer cameras. But it is still a very fine workhorse, when using under very good and bright conditions.
Anyway Photozone should update to D810 or D850 soon, as these are the most used ones for FX and represent the majority of users.
you are wrong.. The D3x is easily surpassed by the D4 in ISO100 in dynamic range and IQ. The Sensor of the D3x is a old Sony, same used in A9 at these days.
So if you compare pictures from both cameras, you will notice, ok 24MP against 16MP is not fair, but IQ of the D4 is superb. The D3x was a niche product
of the crazy race between canon and nikon, as they produced two pro bodies, one for speed, one with higher MP and increased IQ. But the D3x shows age now,
as the sensor is from 2007/2008 and even a consumer body like D750 is better, and even the D600/610 (with no oil on sensor) can easily surpass the D3x in IQ & dynamic range.
D800(e) stands for it's own, and the D3x at least lost here against the D800. D810 again pushed it further with its genious base iso 64.
But yes, the D3x is a fine camera even today, but very slow and some fiddly in LV, not usable at all. As a normal camera in bright light situations,
it can show great results, but again: D4 and D800 show far better results.
Some would say, it is only a D3 with a 24,5MP sensor, and that is, right. Nikon did only add some memory more to make the D3x not so slow,
like it was offered as a buffer upgrade to the D3 and later given to the D3s.
And something we do not have talked about, is the quality of the body itself. Yes here the D3x, and also D3, D3s are clear winners.
Nikon had to bring out a D3xs, with better sensor and self cleaner, faster LV, and some Video, then i would say, that would be a winner camera, and
even good in these days. If you do not believe, take a D3x on a tripod and use LV to make a good shot, then you will know what i mean.
D4 has a far better cpu inside, the expeed3, which is fast and in LV a dream to use, when you come from a D3.